In wireless mobile communications, a codec device is typically used to perform analog-to-digital conversion or digital-to-analog conversion of a baseband signal. Before transmitting the baseband signal, the signal must be amplified. Any DC-offset voltage present in the baseband signal is also amplified, causing signal distortion and saturation. As a result, the DC-offset voltage must be eliminated by determining its level and subtracting that level from the baseband signal.
Early generation cellular phones transmit short bursts of signals. DC-offset compensation is achievable by storing the DC-offset voltage across a capacitor. Since transmission is in short bursts, the DC-offset value is calculated and stored across the capacitor before each burst of data. Problems of capacitor leakage influencing the stored DC-offset may largely be ignored.
Next generation cellular phones, on the other hand, using standards, such as GSM Phase II+ and WCDMA, require the mobile phone to transmit in multiple time slots or in continuous mode. As such, the baseband signal must be transmitted free of any DC-offset for extended periods of time. Continuous operation precludes storing the DC-offset across a capacitor. The DC-offset stored across the capacitor leaks away during the extended period of operation.
It is desirable, therefore, to solve the problem associated with baseband signal offset correction. This invention addresses that problem.